Turbochargers are a type of forced induction system. They deliver air, at greater density than would be possible in the normally aspirated configuration, to the engine intake, allowing more fuel to be combusted, thus boosting the engine's horsepower without significantly increasing engine weight. A smaller turbocharged engine, replacing a normally aspirated engine of a larger physical size, will reduce the mass and can reduce the aerodynamic frontal area of the vehicle.
Turbochargers use the exhaust flow from the engine exhaust manifold to drive a turbine wheel (21), which is located in the turbine housing (2). Once the exhaust gas has passed through the turbine wheel and the turbine wheel has extracted energy from the exhaust gas, the spent exhaust gas exits the turbine housing through the exducer and is ducted to the vehicle downpipe and usually to after-treatment devices such as catalytic converters, particulate traps, and NOx traps.
In a wastegated turbocharger, the turbine volute is fluidly connected to the turbine exducer by a bypass duct. Flow through the bypass duct is controlled by a wastegate valve. Because the inlet of the bypass duct is on the inlet side of the volute, which is upstream of the turbine wheel, and the outlet of the bypass duct is on the exducer side of the volute, which is downstream of the turbine wheel, flow through the bypass duct, when in the bypass mode, bypasses the turbine wheel, thus not powering the turbine wheel. To operate the wastegate, an actuating or control force must be transmitted from outside the turbine housing, through the turbine housing, to the wastegate valve inside the turbine housing. A wastegate pivot shaft extends through the turbine housing and rotates about its axis (64) when driven by an actuator (40). Outside the turbine housing the actuator (40) is connected to a wastegate arm (74) via a linkage (50, 51, 72), and the wastegate arm (74) is connected to the wastegate pivot shaft. Inside the turbine housing, the pivot shaft is connected to a wastegate valve. Actuating force from the actuator is translated into rotation of the pivot shaft, moving the wastegate valve inside of the turbine housing to bypass exhaust flow to the turbine wheel.
Turbine housings experience great temperature gradients and temperature flux. The outside of the turbine housing faces ambient air temperature while the volute surfaces contact exhaust gases ranging from 740° C. to 1050° C. depending on the fuel used in the engine. It is essential that the actuator, via the translated motions described above, be able to control the wastegate to thereby control flow to the turbine wheel in an accurate, repeatable, non-jamming manner. The proximity of a wastegate actuator to the turbine housing has multiple effects. Heat can conductively travel up the actuator shaft (50) to the actuator diaphragm (48). Heat from the turbine housing, to which most actuators are mounted, can be radiatively transferred not only to the actuator shaft and thence to the actuator diaphragm, but also to the actuator canister components such as: the base ring (46), the lower canister (44) and the upper canister (45). The latter components are sometimes protected from radiative heat transfer by incorporating an actuator heat shield (43) surrounding the affected components.
A variable geometry turbine (VTG) mechanism is used not only to control the flow of exhaust gas to the turbine wheel but also to control the turbine back pressure required to drive EGR exhaust gas, against a pressure gradient, into the compressor system to be re-admitted into the combustion chamber. An actuator is used to control the angle of a vane set within the turbine housing and that, in turn, controls the turbine power.
Regulated two stage turbocharger (R2S) configurations have multiple turbos configured such that a flap or valve on one turbine housing can alter the exhaust flow to the second turbine housing. Depending on the requirement, the turbine stages can either be in series, sequential or in a parallel configuration. R2S configurations can be used to control turbine flow and exhaust back pressure, to control EGR flow, or to control a large or a small turbo to suit engine requirements such as transient performance or steady state performance. The valve or flap is driven by an actuator. On R2S turbochargers there are also valves used to bypass compressor outflows to control the swallowing capacity of large and small compressor stages in the same system.
Turbochargers are located in the engine compartment, outside of the engine block and often (for example on in-line straight four or six cylinder engines) are located adjacent to the wheels. Some turbochargers, for example on twin turbo vee engines, are located very low in the engine compartment to keep the engines center of gravity as low as possible and to make the exhaust manifolds to the turbochargers as short as possible. As such, these turbochargers are subjected to road fluids such as water and mud, and materials from grit to anti-ice chemicals, all of which can penetrate the orifices in the turbocharger.
Pneumatic actuators operate by air pressure (which can be positive or negative, typically depending upon the source of the pressure) distending a diaphragm being resisted by a spring of known rate, often accompanied by atmospheric pressure on the spring side of the diaphragm. With respect to the actuator, the difference between a positive pressure and a negative pressure simply being the side of the diaphragm into which the pressure is supplied. The motion of the diaphragm (48) is transferred to the extension of a shaft (50), which then translates to rotation of a wastegate arm (62) mechanically or chemically attached to a wastegate pivot shaft which rotates, opening or closing the wastegate valve. A wastegate spring (47) resists the pressure exerted on the diaphragm and is used to return the shaft to its resting position (with the wastegate valve in the closed position).
For clarity of terms the following definitions have been adopted:                True length of the actuator shaft assembly: the length from the foot of the shaft (50), where the shaft attaches to the upper cup (49) in the actuator, to the pivot center of the joint at the wastegate shaft end.        Centerline of the wastegate arm: the line joining the axis of rotation of the wastegate pivot shaft and the center of the joint.        Radius described by the wastegate arm: the length between the axis of rotation of the wastegate pivot shaft and the center of the joint (78).        Angle through which the wastegate arm rotates: assumed to be the angle between the wastegate valve being fully closed and fully open.        Effective length of the upper pivot arm of the wastegate actuator shaft: the length from the aforementioned joint of the shaft to the upper cup (49) to the guide bearing (53) about which the shaft pivots.        The effective lower pivot arm of the actuator shaft: the length from the guide bearing (53) to the center of the joint which attaches the actuator shaft to the wastegate arm (62).        
It is assumed that: at the mid point of travel of the actuator shaft, the angle between the centerline of the wastegate arm and the centerline of the actuator shaft is 90°, thus minimizing the subtended angle described by the rotation of the wastegate arm.
The angle through which the actuator shaft rocks is determined by a number of relationships:                For a fixed true length of the actuator shaft assembly, the angle subtended by the radius described by the wastegate arm (from wastegate valve open to closed) is a function of the length (i.e., radius) between the axis (64) about which the wastegate arm rotates, and the pivot center (78) of the joint which attaches the wastegate arm to the actuator shaft assembly.        For a fixed true length of the radius described by the wastegate arm, the angle subtended by the wastegate arm is a function of the length of the actuator shaft.        
Since the actuator (43) is mounted fixed to the turbine housing, while the joint between actuator shaft assembly and wastegate arm pivots about pivot center (78), the wastegate shaft assembly pivots about a bearing (53) attached within the lower canister (44). Since it becomes the pivot about which the shaft rocks within the canister, the axial position of the bearing (53) controls the radial position of the actuator piston (49) in response to a radial displacement of the wastegate arm, as determined by an axial displacement of the actuator shaft (50). The closer the axial position of the bearing to the piston, the greater the radial displacement of the piston (for a given wastegate arm rotation). In some actuators the radial position of the bearing (53) is modified by having the bearing's radial location partially controlled by a flexible member such as an “O” ring. This somewhat complicated arrangement minimizes the radial displacement of the diaphragm in an effort to increase the life of the diaphragm.
The actuator component critical to actuator life is the diaphragm. The diaphragm (48) is located in the upper canister shell (45) such that the outer rim of the diaphragm is captured by the joint of the lower canister (44) and the upper canister (45) to produce an airtight seal of the diaphragm and the upper canister shell.
As commanded by the engine control unit (ECU), air pressure is delivered to the actuator through the air fitting (52) to fill the void between the actuator (48) and the upper canister shell (45). The pressure of the incoming air to the actuator forces the diaphragm away from the at-rest position, resisted by the force exerted by a spring (47). The inflation of the void behind the diaphragm forces the diaphragm to compress the spring via displacement of a piston (49) to which is mechanically attached the actuator shaft (50) as described above, while at the same time compressing the space and increasing the pressure on the spring-side of the actuator, which may cause escape of air through a vent or a gap between the actuator shaft and the bearing (53). As the command pressure to the actuator is reduced, the spring forces a return of the piston, reducing pressure on the spring-side of the actuator, leading to a sucking in of ambient air through the bore in the bearing (53).
As described above, the actuator is typically close-coupled to one turbine housing, or a plurality of turbine housings. In order for the actuator to live at the temperatures associated with such proximity to thermal sources the diaphragm is constructed of a composite of fluorosilicone and Kevlar to provide acceptable life at temperature and duty cycle.
Any debris which enters the area in which the diaphragm contacts either the piston or the outer canister shells of the actuator will result in fretting of the diaphragm material which will ultimately lead to failure of the diaphragm and thus the actuator.
The life of a wastegate actuator is compromised by many facets of both design and location: The angle through which the actuator shaft is displaced, the temperature of the critical areas of the diaphragm, the duty cycle, the shape of the components in contact with the diaphragm, and the physical environment to which the actuator is subjected.
Thus it can be seen that there is a need for a protective cover for an actuator shaft to operate in the harsh environment and to accommodate the complex motions of wastegate and VTG actuators in turbochargers.